Methods for the collection of pituitary portal blood have been employed in studies concerned with the detection or measurement of hypothalamic secretion of luteinizing hormone releasing hormone (LHRH) (Fink & Harris 1970; Harris & Ruf 1970: Burger et al. 1972; Fink & Jamieson 1973; Carmel et al 1976; Eskay et al 1975; Sarkar et al 1976) and thyrotropin releasing hormone (TRH) (Wilbur & Porter 1970; Ching & Utiger 1976) in rats. Up until now, the method of portal blood collection has been adequate for the detection of these neurohormones in portal blood. However, the validity of its use for the measurement of hypothalamic secretion of LHRH and TRH has not been established. As a consequence, the present study is designed to determine whether or not measurements of LHRH and TRH in portal blood, collected in the manner reported by Porter and Smith (1967), constitutes a reliable means of assessing hypothalamic secretion of these releasing hormones. In this regard, the aims of this study are to examine the effects of several important parameters that could influence our ability to assess accurately the hypothalamic release of LHRH and TRH. These are: the physiological state of the donor rat, the protocol for the collection of portal blood, recovery of LHRH and TRH from blood for assay, time-related distribution of LHRH and TRH in brain and blood, and method of measuring LHRH and TRH in blood. Following the establishment of the most reliable and valid procedure for the collection of portal blood and quantification of LHRH and TRH therein, physiological studies involving gonadectomy and thryoidectomy, and replacement therapy by the appopriate target gland hormone are to be enacted to further test the methodologies and experimental protocols.